The present invention relates to a scanning electron microscope for obtaining a two-dimensional image of a specimen by scanning a focused electron beam over the specimen and detecting the produced secondary signal. Especially, the invention relates to a scanning electron microscope for measuring patterns formed on substrates such as semiconductor substrates and for observing defects.
Semiconductor devices are fabricated by forming a pattern on a photomask, transferring the pattern onto a wafer by lithography and etching steps, and repeating this sequence of operations. In this manufacturing process, it is important to achieve a good yield quickly and to maintain stable operation of the production process. For these purposes, it is essential that in-line inspection of wafers be performed, found defects be quickly analyzed, causes of generation of the defects be sought for, and the sought causes be utilized for countermeasures. To quickly couple the results of inspection to a countermeasure for preventing defects, automatic defect review and classification technology for reviewing a number of detected defects at high speed and classifying them in terms of causes is a key technology. Furthermore, as the production process has become finer, defect sizes affecting the production yield of semiconductor devices have become finer. It is difficult to review defects at high resolution with an optical review tool. Therefore, defect-review SEMs (scanning electron microscopes) capable of reviewing defects at high speed and high resolution have been commercialized. In recent years, semiconductor devices have been fabricated at increasing scales of integration and with finer features. With these trends, various kinds of patterns have been formed on wafers. It has become more important to evaluate and measure the shapes and dimensions of the formed patterns. In order to measure a multiplicity of measurement points at high speed and stably, metrology SEMs capable of observing and measuring semiconductor patterns at high resolution have been commercialized.
The above-described scanning electron microscopes for inspection of semiconductor devices have a function of acquiring a high-magnification SEM image. In particular, a low-magnification SEM image of an object under observation is first derived. The low-magnification image has a large aperture angle. This image is searched for the object, and the object is brought to the center of the image. Then, the high-magnification SEM image is acquired for detailed observation.
The low-magnification image needs to have a large depth of focus to facilitate searching the image for the object. The high-magnification image needs high resolution to permit detailed observation. JP-A-1-236563 and US2006/0226362A1 (hereinafter referred to as patent references 1 and 2, respectively) describe techniques of obtaining large depths of focus by varying the excitation of the condenser lens to move the image point of the condenser lens and varying the aperture angle of the electron beam impinging on the specimen. Patent reference 1 does not mention of switching of the magnification. Patent reference 2 states that the aperture angle of the electron beam is varied according to the magnification such that a low-magnification image is obtained with a large depth of focus and that a high-magnification image is obtained with high resolution.